1. Field of the Invention
This invention relates to a pneumatic tire, and more particularly to a pneumatic tire for passenger car having a directional tread pattern defined by a plurality of circumferential grooves continuously extending in a circumferential direction of the tire and a plurality of directional slant grooves arranged at given intervals in the circumferential direction.
2. Description of Related Art
In order to efficiently discharge water in a ground contact region to improve running performances in rainy weather, the conventionally typical pneumatic tire for passenger car generally adopts a design technique wherein a plurality of circumferential grooves continuously extending in a circumferential direction and a plurality of directional slant grooves arranged at given intervals in the circumferential direction are arranged in a tread portion to form a directional tread pattern and these circumferential grooves are communicated with the directional slant grooves so as to discharge water overflowed from the circumferential groove toward an outside of the ground contact region.
In the pneumatic tire having a directional tread pattern of excellent wet performances designed by this technique, however, it has been confirmed to degrade a pattern noise as mentioned below.
The pattern noise in the tire is constructed with various factors. One of these factors is a striking sound produced when the tire rotates and contacts with a road surface under loading. When the above design technique of communicating the circumferential groove with the slant groove is applied for discharging water overflowed from the circumferential groove toward the outside of the ground contact region, the tread portion is divided into many small blocks by the slant grooves, so that the movement of these small blocks becomes relatively large during the running of the tire and hence uneven wear produced by friction to the road surface increases. Such an uneven wear produces a step difference between the adjoining blocks, which increases the striking sound produced when the tire contacts with the road surface under loading to degrade the pattern noise in the tire.
It is, therefore, an object of the invention to solve the aforementioned problem of the conventional technique and to provide a pneumatic tire for passenger car having a direction tread pattern defined by a plurality of circumferential grooves continuously extending in a circumferential direction of the tire and a plurality of directional slant grooves arranged at given intervals in the circumferential direction which can control the occurrence of uneven wear to control or prevent the degradation of the pattern noise in the tire.
According to the invention, there is the provision of in a pneumatic tire having a direction tread pattern defined by a plurality of circumferential grooves continuously extending in a circumferential direction of the tire and a plurality of directional slant grooves arranged at given intervals in the circumferential direction, an improvement wherein:
(1) the circumferential grooves are a pair of circumferential center grooves arranged in a central zone of a tread and a pair of circumferential side grooves arranged in both side zones of the tread;
(2) a circumferential center rib continuously extending in the circumferential direction is formed in the central zone of the tread by the pair of circumferential center grooves;
(3) a pair of circumferential middle ribs are formed by the pair of circumferential center grooves and the pair of circumferential side grooves;
(4) a pair of circumferential side ribs are formed by the pair of circumferential side grooves and a pair of tread ends;
(5) the directional slant grooves are directional slant center grooves formed in the circumferential middle rib and directional slant side grooves formed in the pair of circumferential side ribs;
(6) each of the directional slant center grooves is opened to either of the circumferential center groove and the circumferential side groove and extended slantly with respect to an equatorial plane of the tire and terminated in the circumferential middle rib;
(7) each of the directional slant side grooves is opened to the tread end and extended toward the inside of the tread at an angle of 0xcx9c45xc2x0 with respect to a line perpendicular to the equatorial plane and terminated in the circumferential side rib; and
(8) a ratio of number of directional slant center grooves to number of directional slant side grooves is 2:3.
In a preferable embodiment of the invention, a thickness of the tread is gradually decreased from a center of the tread toward each side end of the tread.
The term xe2x80x9cdirectional slant groovexe2x80x9d used herein means a groove extending slantly with respect to the equatorial plane and forming so-called directional tread pattern wherein a rotating direction (forward direction) of the tire is designated so as to firstly contact a slantly extending portion of the groove near to the equatorial plane of the tire with ground and secondly contact a portion thereof opposite to the equatorial plane with ground when the tire is mounted onto a vehicle.